The present invention relates to a method and apparatus for determining the region of a chrominance space in which an input point (a point to be determined) included in an image signal is located.
Various types of image capturing devices and image reproducing devices determine the region of a chrominance space in which an input point for an image signal is located. Based on the determination result, such devices perform image processing for adjusting or converting the color phase of the image signal. It is desirable that the efficiency for such determination be increased.
The determination of the color phase for an image signal in a chrominance space (Cb-Cr space) will now be discussed with reference to FIG. 1. The region (one of A to E) in the Cb-Cr space to which an input point P of coordinates x, y belongs is determined for determining the color phase. Regions A to E are defined by five boundary lines L1 to L5, which extend through the origin O.
FIG. 2 shows a first prior art example of a method for determining the region of the chrominance space to which the input point P belongs. First, an angle αP formed between a line segment, which connects the input point P with the origin O, and a lateral axis (Cb axis) of the Cb-Cr space is calculated. More specifically, from coordinates x, y, tan αP is obtained by y/x. Thus, αP is obtained from tan−1(y/x).
The angle αP is then compared with angles α1 to α5, which are formed between the boundary lines L1 to L5 and the lateral axis. This leads to the determination of which one of regions A to E the input point P belongs to.
FIG. 3 shows the operation of a determination device that performs the above determination. A divider 1 calculates y/x based on the coordinates x, y of the input point P and provides the coordinates x, y to a calculation section 2. The calculation section 2 computes tan−1(y/x) to obtain the angle αP formed between the input point P and the lateral axis and provides the angle αP to comparators 3a to 3e. 
The comparators 3a to 3e compare the angle αP with the angles α1 to α5 and provides the comparison result to a determiner 4.
Region codes corresponding to regions A to E are input to the determiner 4. Based on the comparison results of the comparators 3a to 3e, the determiner 4 outputs the region code corresponding to the region to which the input point P belongs. Then, based on the region code output from the determiner 4, a predetermined adjustment is performed on the color phase of the input point P.
A second prior art example of a method for determining the region in the chrominance space to which the input point P belongs will now be described with reference to FIG. 4.
First, an outer product of the coordinates x, y of the input point P and vectors (x1, y1 to x5, y5) of each of the five boundary lines L1 to L5, which define regions A to E, are computed. For example, with regard to the vector x1, y1 of the boundary line L1 and the coordinates x, y of the input point P, the value of (x×y1)−(y×x1) is computed. In the same manner, with regard to the vectors L2 to L5, the values of (x×y2)−(y×x2), (x×y3)−(y×x3), (x×y4)−(y×x4), and (x×y5)−(y×x5) are computed.
An outer product having a negative value indicates that the input point P is located in a right semicircle of the corresponding one of the boundary lines L1 to L5. An outer product having a positive value indicates that the input point P is located in a left semicircle of the corresponding one of the boundary lines L1 to L5. The determination of the one of the regions to which the input point P belongs is determined in accordance with the values of the five outer products.
FIG. 5 shows the operation of a determination device used in the second prior art example. Multipliers 5a to 5j perform the above multiplications. Subtracters 6a to 6e perform the above subtractions and provide the results to comparators 7a to 7e. 
The comparators 7a to 7e compare the computation results of the corresponding subtracters 6a to 6e and provide the comparison results to a determiner 8. Based on the comparison results of the comparators 7a to 7e, the determiner 8 outputs the region code corresponding to the region to which the input point P belongs. Then, based on the region code output from the determiner 8, a predetermined adjustment for the color phase of the input point P is performed.
Japanese Laid-Open Patent Publication Nos. 10-257334 and 2001-285655 describe defining chrominance space regions with lines extending through the origin and determining the color phase based on the region. Japanese Laid-Open Patent Publication No. 9-247701 describes performing color correction for each region defined in the chrominance space. Japanese Laid-Open Patent Publication No. 11-341507 describes a color determination in the Cb-Cr space and a color signal correction process that is based on the determination result.